In each of United Kingdom Patent Specification Nos. 1078791 and 1525861, there is described a motor vehicle transmission system comprising continuously variable gearing driven by the vehicle engine, and split path gearing driven by the continuously variable gearing, the split path gearing having an output shaft which provides drive to the vehicle wheels. In order to positively disconnect the engine from the vehicle wheels, the split path gearing can include means for disconnecting the drive from its output shaft.
In such a transmission system, at a particular ratio setting of the continuously variable gearing, known as "geared neutral", there is no rotation of the output shaft of the split path gearing. An increase or a decrease of the ratio of the continuously variable gearing from geared neutral results in the vehicle being driven backwards or forwards. When a vehicle is stationary, the continuously variable gearings is set at geared neutral and this setting is then normally maintained. When it is desired to drive the vehicle, the ratio of the continuously variable gearing is displaced from this setting. Alternatively, when the vehicle is stationary, the means for disconnecting drive from the output shaft of the split path gearing may be operated in order to provide a positive disconnection between the engine and the wheels. This is usually achieved by putting the vehicle gear lever into the neutral or park position.
It has been found that when the disconnection means is operated, the ratio of the continuously variable gearing drifts away from the setting which corresponds to geared neutral. Consequently, when drive is subsequently restored to the output shaft of the split path gearing, a transient torque is experienced in the transmission system due to the inertia of the transmission parts and the limits on rapid ratio change imposed by constraints in the continuously variable gearing. Also, if a vehicle is permitted to move with the disconnection means operated, as may be the case where a vehicle is permitted to roll down hill, the ratio setting of the continuously variable gearing will not adjust to the change in the output shaft speed of the split path gearing and consequently the transient torque will be experienced when drive is restored to the output shaft. Such transient torques are unsatisfactory as they reduce the smoothness of the transmission system.
Accordingly, it is an object of this invention to provide a new or improved transmission system and also a method for controlling a transmission system in which the above mentioned disadvantage is overcome or reduced.